Carbohydrates: significance for energy balance and gastrointestinal function

The 10th edition of the RDA discusses carbohydrates in the context of dietary fiber and digestible carbohydrates, which provide energy, and recommends that more than half the energy requirement beyond infancy be provided by carbohydrates and that a desirable intake of fiber be achieved by consumption of fruits, vegetables, legumes, and whole-grain cereals. The recommendations on digestible carbohydrates are primarily based on the desirability of limiting the intake of fat. An additional consideration is the suggestion that excess calories from carbohydrate vs. fat are metabolized and stored with different efficiencies. The scientific basis for recommending an appropriate intake of dietary fiber is not clearly evident in the current RDA. The association between a high plant food, and thus fiber intake, and lower incidence of cardiovascular disease, colon cancer, and diabetes is of interest but difficult to use to form the basis of specific recommendations for an appropriate amount of fiber to include in the diet. The importance of fiber for normal gastrointestinal function provides the best scientific basis for developing recommended intake levels. Dietary fiber, as well as non-digested starch, are the only known dietary components that increase stool weight and are the primary substrates for the microflora in the large intestine. Metabolism of this microflora, including production of short chain fatty acids that are used by epithelial cells, appears to be important for intestinal health. However, the challenge we still face is to define the parameters of gastrointestinal function that are compatible with health.